The present invention relates generally to improvements in playing piano music electromechanically, and more particularly to an improved method and apparatus for producing variable intensity in a piano performance to create expression effects.
It has been known for many years that a piano performance can be recreated by moving the keys and pedals mechanically. In earlier versions, a perforated paper roll was the recording medium, and actuation of the keys and pedals in response to perforations in the roll was achieved by pneumatic means. More recently, magnetic and optical recording media such as magnetic tape and/or magnetic or optical disks are used as the recording media, with solenoids or other electromagnetic devices being used as actuators for the keys and pedals.
When a performance is to be recorded, the piano is played by a musician, and sensors detect the timing and velocity with which the keys are depressed and the hammers associated with the keys are moved. This information is stored digitally on a recording medium such as magnetic tape. When the performance is to be recreated, the digital information is retrieved from the magnetic tape and converted to control signals that energize solenoid actuators to move the keys in the same order and with the same intensities as in the original performance.
To a large degree, the unique and satisfying aspects of a musical performance played on a piano are related to the intensities of the individual notes that comprise the performance. If the intensities of the individual notes are correct, the overall effect will be that of a pleasing musical whole, with each note playing its role in the larger musical structure. However, if the intensities of the individual notes are incorrect, the resulting performance will have an unmusical quality. It is due to such incorrect intensity control that performances recreated by inferior instruments are often dismissed as being "mechanical" and therefore undesirable.
Basically, in a reproducing piano, note intensity control is achieved by varying the drive applied to the key actuators. The actuators are typically large and consequently relatively slow, so that the desired drive may be approximated by rapidly alternating between full drive and no drive. The delay inherent in the actuators tends to smooth the rapidly alternating applied drive, and to a first approximation the actuators respond only to the average value of the applied drive.
In U.S Pat. No. 4,132,141, such alternating applied drive is achieved by creating a sequence of pulses of substantially fixed repetition rate, and varying the width of the pulses such that the average drive voltage specified by the pulses corresponds to the desired drive voltage. While this approach controls the drive, it suffers from several deficiencies that make it unattractive for use in a high-performance, low-cost instrument. One deficiency derives from the fact that the pulse width is modulated according to the desired drive. In order for the average drive to be controlled in this way, the height of the pulses must be uniform. Since the height of the pulses mirrors the actuator supply voltage, this supply voltage must remain constant for proper control to be achieved. A single unregulated supply is normally used for all of the actuators in the interest of economy, and its output voltage drops when many notes are played concurrently. As a result, a regulated power supply would be required to achieve the desired control, but this approach would add unnecessary cost to the instrument.
Another deficiency encountered in pulse width control schemes appears during soft play, which requires the application of a relatively low average drive voltage. The accuracy of control for soft play is compromised by the very narrow pulses that occur when low drive is required. For such very narrow pulses, the switching times constitute a significant fraction of the pulse width, resulting in unpredictable behavior.
A still further deficiency of pulse width control schemes arises from the fact that pulse width modulators are complex and therefore expensive. Ideally, there should be individual control of the drive voltage applied to each note solenoid. While this can be done with pulse-width modulation, the provision of one pulse-width modulator per note results in a system that is unnecessarily complex and expensive.
Accordingly, there has been a need for a novel method and apparatus of simplified and relatively inexpensive construction for producing variable intensity in a piano performance. Such an apparatus and method are needed which yield excellent drive control even at very low drive levels, individually control the drive to each note solenoid, and compensate for supply voltage variations. The present invention achieves these needs and provides other related advantages.